1. Field of the Invention
The present invention relates to a ball screw mechanism and an electrically powered steering device utilizing such ball screw mechanism.
2. Description of the Prior Art
The electrically powered steering device for an automotive vehicle is a device for assisting a steering force of the steering wheel by means of an electric motor and is available in various types. One of them is a model designed not only to apply an axially shifting force to a retractable steering shaft drivingly coupled with a steering mechanism for wheels by translating a rotation of the steering wheel by means of a motion translating mechanism such as a rack-and-pinion mechanism but also to apply an output of an electric motor as an axially shifting force to the retractable steering shaft through a ball screw mechanism. In order to render the electrically powered steering device to be assembled compact in size and, hence, to the automotive vehicle to be lightweight in its entirety, the ball screw mechanism is desired to have a compact size and a high load capacity.
The ball screw mechanism is also available in various types depending on the mode of circulation of a series of balls, including a bridge type ball screw design. The prior art bridge type ball screw mechanism is shown in FIGS. 13A to 13C, reference to which will now be made. A ball screw shaft 51 has its outer peripheral surface formed with an externally threaded helical groove 52 and has a rotary nut 53 mounted thereon. This rotary nut 53 has an inner peripheral surface with an internally threaded helical groove 54 cooperable with the externally threaded helical groove 52 and is threadingly engaged with the ball screw shaft 51 through a series of balls 55 that are received in part in the externally threaded helical groove 52 and in part in the internally threaded helical groove 54. A cylindrical wall defining the rotary nut 53 has a plurality of generally elliptical mounting holes 56 each extending completely across the thickness of the cylindrical wall of the rotary nut 53 having its opposite ends aligned respectively with the neighboring convolutions of the externally threaded helical groove 52. These elliptical mounting holes 56 are closed by a similarly elliptically shaped bridge member 57 that is fixedly plugged therein. The respective bridge member 57 has a concave surface where a connecting groove 58 is defined so that the neighboring convolutions of the internally threaded helical groove 54 are communicated with each other to thereby define a ball rolling passage along which the series of the balls 55 rollingly traverse from one of the convolutions of the internally threaded helical groove 54 to the next adjacent convolution of the same internally threaded helical groove 54. Thus, the series of the balls 55 movably interposed between the internally and externally threaded helical grooves 52 and 54 can rollingly move along and between the internally and externally threaded helical grooves 52 and 54 and are then guided along the connecting grooves 58 in the bridge members 57 so as to ride over corresponding threads of the ball screw shaft 51 from one convolution of the externally threaded helical groove 54 onto the next adjacent convolution of the externally threaded helical groove 54.
The bridge type ball screw mechanism has an advantage in that the rotary nut 53 can have a reduced outer diameter, but has a disadvantage in that because of the plural bridge members 57 necessitated the number of component parts forming the ball screw mechanism is large. Also, if an attempt is made to increase the load capacity, the bridge type ball screw mechanism tends to pose the following problems.
While one of the following means for increasing the load capacity is available in the ball screw mechanism, the bridge type ball screw mechanism cannot employ any of those means because of the reason described therein.
(1) To reduce the pitch to make it possible to increase the number of balls to be circulated. Where this means is employed, the use of the balls of a relatively small diameter results in decrease of the load capacity. Because of this, it is necessary to reduce the pitch without the ball diameter being altered. However, with the bridge type ball screw mechanism, it is not possible to reduce the pitch without the ball diameter being altered, because one bridge member 57 is necessitated for a single pitch (the span between the neighboring helical grooves).
(2) To form a groove between the successive leads to thereby form a multi-thread screws so that an effect similar to that afforded when the number of the balls to be circulated is increased. In the case of the multi-thread screw, the lead (the distance of movement per rotation) is limited and, therefore, the lead cannot be reduced. Also, although the multi-thread screw can be employed in an end-cap type ball screw mechanism, the bridge type ball screw mechanism cannot employ the multi-thread screw because as discussed in the previous paragraph one bridge member 57 is necessitated for a single pitch.
As discussed above, where the load capacity is desired to be increased in the bridge type ball screw mechanism, there is no way other than to employ the rotary nut having an increased length so that an increased number of the balls can be employed and, accordingly, while the bridge type ball screw mechanism is advantageous in that the rotary nut of a relatively small outer diameter can be employed, the use of the rotary nut of the increased length hampers compactization of the bridge type ball screw mechanism as a whole. Also, the use of the rotary nut of the increased length requires a corresponding increase of the number of the bridge members 57 used and, therefore, the number of machining processes and the number of component parts tend to increase, resulting in increase of costs required to manufacture the bridge type ball screw mechanism.
To alleviate the foregoing problems, the assignee of the present invention has filed the Japanese patent Application No. 11-313518 and the U.S. patent application Ser. No. 09/704,678, in which it is suggested a bridge member 57A having a plurality of connecting passages 58 defined therein as shown in FIGS. 14 and 15. The bridge member 57A disclosed therein has opposite side edges spaced in a direction circumferentially of the rotary nut 53, which edges are formed with respective guide walls 68 upstanding therefrom in a direction radially outwardly of the rotary nut 53. These guide walls 68 are each formed with a separation preventive projection 68a at a free edge thereof such that when the bridge member 57A is mounted in position within a mounting hole 56A defined in the rotary nut 53, the separation preventive projections 68a integral with the respective guide walls 68 are resiliently engaged against associated steps 69 formed on opposite side faces of the mounting hole 56A. By this engagement, the bridge member 57A is retained firmly within the mounting hole 56A in the rotary nut 53.
However, it has been found that where the guide walls 68 having the separation preventive projections 68a are integrally formed with the bridge member 57A, the guide walls 68 are prone to a considerable deformation and a difficulty in manufacture because each of the guide walls 68 is thin walled having a considerably small wall thickness.
Accordingly, the present invention is intended to provide a compact ball screw mechanism having a minimized number of component parts and also having a high load bearing capacity, wherein the bridge member of a simplified structure can easily, but firmly be fitted to the rotary nut.
The present invention has another object to facilitate formation by molding of the bridge member by simplifying the structure of the bridge member.
An additional object of the present invention is to provide an electrically powered steering device employing the ball screw mechanism of the type referred to above for transmitting an output of an electric motor, which mechanism is compact in size and easy to manufacture and having a high load capacity so that the device as a whole can be assembled compact in size.
In order to accomplish these objects of the present invention, in a first aspect of the present invention there is provided a ball screw mechanism which includes a ball screw shaft; a rotary nut having an internally threaded helical groove defined on an inner peripheral surface thereof in face-to-face relation with the ball screw mechanism; a plurality of balls rollingly accommodated in series within a ball rolling passage defined between the ball screw shaft and the internally threaded helical groove for transmitting a force from one of the rotary nut and the ball screw shaft to the other thereof, and at least one bridge member having at least one groove defined therein and mounted in the rotary nut with the groove communicating neighboring convolutions of the internally threaded helical groove. The bridge member is fixedly connected with the rotary nut by means of a plastic deformation.
According to this structure of the present invention, the bridge member is fixedly integrated with the rotary nut by the utilization of the plastic deformation. Accordingly, a simplified structure can be employed to integrate the bridge member with the rotary nut. Specifically, since the bridge member is allowed to undergo plastic deformation, the bridge member need not be provided with complicatedly shaped elements for engagement and can therefore be simplified in shape. For this reason, molding can easily be performed where the bridge member is to be formed of a sintered metal by the use of an injection molding technique.
Also, where the bridge member is formed with a plurality of connecting grooves, the pitch of the internally threaded helical groove can be reduced such that without the length of the rotary nut being increased, the number of the balls to be circulated can be increased to thereby increase the load bearing capacity. Because of this, together with the advantage of a reduced outer diameter of the rotary nut as is the case with that used in the conventional bridge type ball screw mechanism, it can have a compact size and an increased load capacity. Moreover, by forming the plural connecting grooves in one bridge member, the number of the bridge members to be employed can advantageously be reduced, resulting in a minimized number of component parts and ease to assembly and, accordingly the cost of manufacture can advantageously be reduced. In addition, the use of the plural connecting grooves in the single bridge member facilitates increase of the preciseness.
In the practice of the present invention, the bridge member may include an arm engageable with the internally threaded helical groove of the rotary nut for positioning the bridge member axially relative to the rotary nut.
By allowing the arm integral with the bridge member to engage in the internally threaded helical groove formed in the rotary nut as a ball rolling face, the bridge member can be highly accurately positioned relative to the rotary nut.
Also, in the practice of the present invention, the rotary nut may have at least one mounting hole defined therein for accommodating the bridge member and, on the other hand, side edges of the bridge member that are opposite to each other in a direction circumferentially of the rotary nut may be formed with respective guide walls upstanding therefrom in a direction radially of the rotary nut. In this structure, the guide walls are crimped to engage respective inner side faces of the mounting hole defined in the rotary nut to thereby allow the bridge member to be fixedly retained within the mounting hole.
The provision of the guide walls in the bridge member and the use of the crimping technique to crimp the guide walls to fix the bridge member relative to the rotary nut effectively facilitates fixture of the bridge member. In particular, the use of the crimping technique effectively eliminates the need to form the separation preventive projections at the free ends of the respective guide walls, resulting in simplification in shape of the bridge member. Because of this, where the bridge member is made of the sintered metal by the use of an injection molding technique, the molding can easily be accomplished.
In a second aspect of the present invention, there is provided a ball screw mechanism which is similar to that according to the first aspect of the present invention, but differs therefrom in that in place of the bridge member that is fixed to the rotary nut by the utilization of the plastic deformation, a separate fixing member is mounted on the bridge member. This separate fixing member includes guide walls upstanding therefrom in a direction radially of the rotary nut along opposite side edges of the rotary nut with respect to a circumferential direction thereof. These guide walls are resiliently engaged with opposite inner side faces of the mounting hole defined in the rotary nut.
According to this second aspect of the present invention, the guide walls of the separate fixing member mounted on the bridge member resiliently engage with the circumferentially spaced inner side faces of the mounting hole defined in the rotary nut to firmly secure the bridge member to the rotary nut. The use of the fixing member separate from, but mounted on the bridge member is effective to simplify the shape of the bridge member. Because of this, where the bridge member is made of a sintered metal by the use of an injection molding technique, the molding thereof can easily be accomplished.
Where the fixing member separate from the bridge member is employed, the separate fixing member may be prepared from a steel plate by means of a press work and is mounted on the bridge member by means of crimping the bridge member. Using the separate fixing member formed from a steel plate by the use of the press work makes it possible to mass-produce the fixing members at a low cost. Also, mounting of the fixing member on the bridge member by the use of a crimping technique facilitates integration of the fixing member with the bridge member.
Where the fixing member separate from the bridge member is employed, the fixing member may be prepared from a steel plate by means of a press work and is mounted on the bridge member by utilization of a resiliency of the fixing member.
Where the separate fixing member is formed from a steel plate by the use of the press work, it is possible to utilize the resiliency of the fixing member to mount the fixing member on the bridge member easily. Mounting of the fixing member by the utilization of its own resiliency can be accomplished if, for example, the fixing member is of a generally U-shaped configuration so that it can be fitted in a fashion sandwiching the bridge member.
According to a third aspect of the present invention, there is provided a ball screw mechanism which is similar to that according to the second aspect of the present invention, but differs therefrom in that in place of the separate fixing member employed in the second aspect, the mounting hole in the rotary nut has engagement steps defined in side faces of the mounting hole that are opposite to each other in a circumferential direction of the rotary nut and, on the other hand, the bridge member is formed with engagement grooves aligned and cooperable with the respective engagement steps, and a fixing member made up of a wire is utilized to engage in part with the engagement steps and in part within the engagement grooves to thereby allow the bridge member to be fixedly retained within the mounting hole.
According to this design, by allowing the fixing member in the form of a wire to be engaged in part within the engagement grooves in the bridge member and in part with the engagement steps in the rotary nut, the bridge member can be fixedly retained by the rotary nut. Even this design makes it possible to simplify the shape of the bridge member since the fixing member is separate from the bridge member. Because of this, where the bridge member is made of a sintered metal by the use of an injection molding technique, the molding can be easily accomplished.
In the structure according to any one of the first to third aspects of the present invention, the bridge member is preferably made of a sintered alloy. The use of the sintered alloy as material for the bridge member makes it possible for the bridge member to be manufactured by the use of injection molding and sintering techniques with no need to use any mechanical machining such as a grinding or a milling, resulting in a good mass-productivity. Accordingly, inexpensive manufacture is possible. Also, even in any one of the second and third aspects of the present invention, the plural connecting grooves may be formed in the single bridge member as is the case with that in the first mentioned aspect of the present invention.
The present invention also provides an electrically powered steering device which includes a housing; a steering shaft drivingly coupled with a steering mechanism for steering wheels; a motion translating mechanism for converting a rotational force of a steering wheel into a force required to displace the steering shaft in a direction axially thereof, a ball screw mechanism including a ball screw shaft defined by a portion of the steering shaft and a rotary nut operatively mounted on the ball screw shaft; and an electric motor for driving the rotary nut. The ball screw mechanism employed in this electrically powered steering device is of the structure described in connection with any one of the first to third aspects of the present invention.
In this electrically powered steering device, the ball screw mechanism employed therein for transmitting the output of the electric motor is compact in structure and has a high load capacity and, therefore, the electrically powered steering device itself can be assembled compact in size.